Combined cycle power plants with turbomachines and heat recovery steam generators (HRSG) are connected to electrical grids that require flexible operation to meet changing power demands. Some power grid cycles command the turbomachines to stop during low demand periods and restart soon thereafter to meet electric power demands of the grid. During operation of the turbomachine and HRSG, the various components expand and contract. For example, thermal expansion may occur due to the relatively high temperature associated with turbine operation, and mechanical expansion may occur due to centripetal forces associated with rotation of the interior components.
Turbomachine components expand and contract at different and varying rates. The varying rates result from component differences in material, geometry, location, and purpose. To accommodate for the discrepancy in expansion and contraction rates, a clearance is designed into the turbomachine between the tips of the blades and shroud, typically referred to as tip clearance. The tip clearance reduces the risk of turbine damage by permitting the blades to expand without contacting the shroud. However, the tip clearance substantially reduces the efficiency of the turbine by permitting a portion of the heated gas to escape past the blades without performing useful work, which wastes energy that would otherwise be available for extraction. A similar clearance may be designed into the compressor between the compressor blades and the compressor case, which may permit air to escape past the compressor blades without compressing.
The size of the tip clearance may vary over stages in an operational cycle of the turbomachine, due to varying thermal and mechanical conditions in the turbomachine. The turbomachine is typically initiated from a “cold start” by increasing the rotor speed and subsequently drawing a load, which effects the clearance between the tips of the turbine blades and the turbine shroud. The turbomachine may then be shutdown for a brief period, such as to correct an issue or due to power demands. During shutdown, the load may be removed, the rotor speed may be reduced, and the components may begin contracting and cooling. Subsequently, a “hot restart” may occur, wherein the turbomachine is restarted before the components return to cold build conditions.
Tight tip clearances observed during the hot restart cycle may be due in part to the turbomachine cooling relatively faster on the exterior (stator) than the interior (rotor) during shutdown. For example, the interior components of the turbine may remain warm, while the stator case may cool and contract toward the interior. The cooling of the stator case may be exacerbated by induced cooling air flow traveling along the length of the turbomachine during shutdown. The turbomachine may have a series of inlet guide vanes positioned along the compressor, which permit air to enter the turbomachine for compression and subsequent expansion. Because these inlet guide vanes may remain at least partially open during shutdown, air may continue to pass into the compressor. The induced air travels along the length of the turbomachine, with flow being supported by the angular momentum of the rotor, and may continue rotating the rotor before entering the HRSG. The induced draft may further cool the stator case during shutdown, thereby resulting in tighter clearances on hot restart.
During shutdown, ambient air infiltration can be naturally induced through the compressor and hot gas path by natural convection of the hot gas contained in the turbomachine, HRSG, flue gas stacks, and due to the pressure differences caused by the wind speed and wind direction at the turbomachine inlet. This ambient air infiltration cools the turbomachine and HRSG which is detrimental to a quick restart due to temperature operational constraints imposed by the turbomachine and HRSG.
Thus, in order to allow restarting of the turbomachine and HRSG as quickly as possible, the induced draft through the turbomachine and HRSG can be actively controlled. Traditionally, in order to counteract the draft, the variable inlet guide vanes of the turbomachine compressor (i.e. the vanes provided at the inlet of the compressor to control the air flow through the turbomachine) are closed and/or intake dampers and louvers (provided e.g. in the inlet section upstream of the compressor) and/or stack dampers (provided e.g. at the stack) are closed. The traditional approach reduces the natural draft through the turbomachine and possibly the heat recovery steam generator, but because of leakages there can still be a substantial amount of induced natural draft.